Non-baryonic dark matter

3.2 Non-baryonic dark matter

The non-baryonic candidates are classified as either hot or cold dark matter. Hot implies the matter was relativistic in the early universe. An example of hot dark matter is the neutrino. From Table 1 it can be seen that the standard model still allows for a small HDM component. The neutrino would fulfill this if it had a non-zero mass, as suggested by recent experimental results from Super-Kamiokande [51 , 71 ]. However, most of the dark matter will be cold dark matter (CDM). The best motivated of the various suggested candidates are two particles that were already invoked for other reasons. These are the axion and the lightest supersymmetric particle (neutralino), which is a member of the generic family of weakly interacting massive particles (WIMPs).

Axions are particles invoked to resolve the strong CP violation problem [75 ]. Without these particles, the level of CP violation seen in the $K0$ decay would give rise to a neutron electric dipole moment in excess of the limits already established. The allowable mass range for the axion is constrained by astrophysical arguments to $10 −3$ to $10 −6 eV∕c2$ .

WIMPs are naturally predicted in supersymmetry theories in which a higher level symmetry is obtained in the particle families by introducing new particles to match each of the known particles. In the so-called minimal supersymmetry models (MSSM), the lightest supersymmetric particle (LSP) is likely to be a neutralino $χ$ , which is a mixture of two neutral gauginos and two neutral higgsinos [79 ]:

There are numerous parameters required to specify a particular MSSM configuration. It turns out that there is a wide range of parameter space in which the production and annihilation rates in the early universe are such that $ΩLSP ≃ Ωcdm$ [45

]. The mass range for neutralinos is $2 2 46 GeV ∕c ≤ m χ ≤ 2000 GeV ∕c$ , where the lower limit comes from accelerator data from LEP [66